The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is a factor in engine design and engine control. Oxides of nitrogen, NOx, are known by-products of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reductions of NOx created in the combustion process and management of NOx in an exhaust aftertreatment system are desirable.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas feedstream temperatures and engine air/fuel ratio. Additionally, aftertreatment devices include materials, such as catalyst beds, prone to damage or degradation as a result of use over time and exposure to high temperatures.
Engine control methods may utilize diverse operating strategies to optimize combustion to generate work. Some operating strategies for optimizing combustion include lean, localized, or stratified combustion within the combustion chamber in order to reduce a fuel charge operating in an unthrottled condition to reduce air intake pumping losses. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas feedstream can be greatly reduced from peak values. The reduced heat energy can be challenging to exhaust aftertreatment strategies, since aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas feedstream temperature, to operate adequately to treat NOx emissions.
Aftertreatment devices effect chemical reactions to treat exhaust gas feedstream. One exemplary aftertreatment device includes a selective catalytic reduction device (SCR). Known SCR devices utilize ammonia derived from urea injection to treat NOx. Ammonia stored on a catalyst bed within the SCR reacts with NOx, preferably in a desired proportion of NO and NO2, and produces favorable reactions to reduce NOx. Since urea injection devices require participation by the owner in monitoring and maintaining fluid levels, passive SCR devices are preferred because they do not employ urea injection devices. Passive SCR devices rely on a catalyst, such as a three-way catalyst, to convert exhaust derived from a rich combustion process to ammonia for storage on a bed of the SCR. The passive SCR devices utilize the ammonia to reduce NOx.